c Swinburne University of Technology, 2010 31 OF 1 AGE P : Time and Tide Credit: Ruth Maddison Moon HET602-M04A02: Watching the Moon: Time and Tide Watching the c Swinburne University of Technology, 2010 31 OF 2 AGE P HET602-M04A02: Watching the Moon: Time and Tide the evolution of the length of days and months on Earth. tidal forces, synchronous rotation of the Moon, and • • • In this Activity, we will investigate Summary c Swinburne University of Technology, 2010 31 OF 3 AGE P : NASA/NSSDC Credit tidal . gravity . HET602-M04A02: Watching the Moon: Time and Tide effects We tend to talk about theMoon effects of as gravity if on they the are Earth pointfinite and objects size, the – and but this they are brings not: about interesting each and has a important As the Moon travels in its almostit circular too path around is the in Earth, free fall under Tidal forces c Swinburne University of Technology, 2010 31 OF 4 AGE P black (particularly Jupiter). Tidal effects are also crucial in the regions surrounding planets . We’ll meet some of these other examples of tidal effects in later Modules - for now, we will HET602-M04A02: Watching the Moon: Time and Tide holes concentrate on the Earth-Moon system. Instead of starting with oceanon tides the on Moon. Earth, we’ll look at an easier example to explain - tidal forces We usually think of thewidespread. Earth’s oceans when we talkThe about tides Moon - as but well tidalother effects as are the much more Earth experiences sizable tidal forces, as do the natural satellites of c Swinburne University of Technology, 2010 31 OF 5 AGE P . orbit keeps it travelling along its almost circular centre : Earth by: Dr. Edwin V. Bell, II (NSSDC/Raytheon ITSS) Not to scale! Credit acting on the Moon’s F HET602-M04A02: Watching the Moon: Time and Tide The force of gravity c Swinburne University of Technology, 2010 31 OF 6 AGE P 2 1 R ∝ grav F squared, Not to scale! distance HET602-M04A02: Watching the Moon: Time and Tide this means that the forcethe of Moon. gravity is stronger on the near side of the Moon than on the far side of However, the force of gravity acts on allto parts the of inverse the of Moon. Because the force of gravity is proportional c Swinburne University of Technology, 2010 31 OF 7 AGE P radius cubed: orbital 3 R squared is proportional to the ∝ 2 P orbital period at larger radii have lower orbital speeds, and vice versa. This comes about because orbits HET602-M04A02: Watching the Moon: Time and Tide So stable the force of gravity decreases rapidly with distance. According to Kepler’s third law, the c Swinburne University of Technology, 2010 31 OF 8 AGE P : Moon by: NASA/NSSDC Credit - a point on the side facing Earth, - a point at the centre of the- Moon, a and point on the side furthest from Earth. HET602-M04A02: Watching the Moon: Time and Tide A B C • • • Let’s consider the motion of three points on the Moon: c Swinburne University of Technology, 2010 31 OF 9 AGE P : Moon by: NASA/NSSDC Credit . B would travel faster A is so slow compared to its axis would travel more slowly than were free to travel in independent orbits around C C - have to travel around the Earth at the same orbital C and , and . HET602-M04A02: Watching the Moon: Time and Tide B and B , B A , orbital motion around the Earth that we can ignore it for now.) the Earth, according to Kepler’s third law than speed If (The Moon’s rotation on its However, because they are physically joined,A all three points - c Swinburne University of Technology, 2010 31 OF 10 AGE P : Moon by: NASA/NSSDC Credit is closer to the Earth, and so is travelling too slowly for is further from the Earth, so it is travelling too fast for an is travelling at the right speed for its orbit, HET602-M04A02: Watching the Moon: Time and Tide independent orbit. B A an independent orbit, and C • • • So for each point: c Swinburne University of Technology, 2010 31 OF 11 AGE P : Moon by: NASA/NSSDC Credit were not fixed to the Moon: C and would move in towards the Earth (as it is not travelling would shift out further from the Earth (as it is travelling would continue on its original obit (as it is travelling at the B HET602-M04A02: Watching the Moon: Time and Tide , C too fast to maintain its present orbit). A fast enough to maintain its present orbit), B right speed for its present orbit), and A • • • If c Swinburne University of Technology, 2010 31 OF 12 AGE P : Moon by: NASA/NSSDC Credit , aligned tidal bulge bulge HET602-M04A02: Watching the Moon: Time and Tide Tidal roughly towards Earth. The Moon, however, isgravity a and chemical solid bonds. object, held togetherSo the by differential both gravitational pullnear of and the far Earth sides on only the distortThe Moon’s its Moon shape. does indeed have a small c Swinburne University of Technology, 2010 31 OF 13 AGE P comet objects such as the orbiting on the near and far sides of is greater and the Moon is smaller in size and has a weaker surface mass act on the Earth too. The Earth’s solid crust is tidally distorted by the Moon, gravitational force tidal forces HET602-M04A02: Watching the Moon: Time and Tide Shoemaker-Levy 9). These causing movements in the Earth’s surface of upBecause to the about 20 Earth’s cm. gravity (i.e. things weigh less onabout the 20 Moon), times the stronger that tidal the forceslarger tides generated tidal that by bulge the the than Moon Earth the raises on on Earth the the does. Moon Earth. are Thus the Moon has a much Moon tend to distort these objects (and can even pull loosely bound objects apart, as with Differences in the c Swinburne University of Technology, 2010 31 OF 14 AGE P lead to bulges . day HET602-M04A02: Watching the Moon: Time and Tide If the Earth were uniformlyeffect covered of in continents and ocean, bays we in wouldEarth obstructing expect and have tides channeling only water of one flow tide a means per few that day, metres. some while locations others However, on the have tides that are up to 15 m high! two ocean high tides (and low tides) each As we know, the Earth’s oceans are strongly distorted by tidal forces. The two tidal c Swinburne University of Technology, 2010 31 OF 15 AGE P also produces tidal effects on Earth, but only at about half the magnitude of those due to HET602-M04A02: Watching the Moon: Time and Tide Sun the Moon. How can thisthan be, that given due that to the the gravitational Moon? force onThink the about Earth this due for a to moment the before Sun moving is on greater to the next slide. The c Swinburne University of Technology, 2010 31 OF 16 AGE P tide raising force is proportional to the inverse distance cubed, while the gravitational HET602-M04A02: Watching the Moon: Time and Tide differential This distance is from centre to centre. force is proportional to the inverse distance squared: The c Swinburne University of Technology, 2010 31 OF 17 AGE P 3 ) Sun kg − km km kg Sun 8 22 5 30 M 10 10 10 Earth 10 D × × × ( × 498 348 99 844 . . . . and = 1 = 7 = 1 = 3 3 ) Sun Sun − Moon Moon M − M Moon − Earth Moon Earth D M D Earth D ( HET602-M04A02: Watching the Moon: Time and Tide So even though the massout of that the the Sun tidal force is of obviously the a SunIn lot on case bigger the you Earth want than to is the try about mass the half of numbers the the for tidal yourself: Moon, force of it the works Moon on the Earth. and you need to compare c Swinburne University of Technology, 2010 31 OF 18 AGE P (when the ocean ‘springs up’ - nothing to do spring tides Not to scale! HET602-M04A02: Watching the Moon: Time and Tide So even though the Suntidal effects exerts are a not as greater great. gravitational But force theThe on Sun’s tidal largest the effects ocean can Earth tides be then seen on to does Earth, effect the the called Earth’s Moon, ocean its tides: with the season!) occur when the Sun, Earth and the Moon are aligned: c Swinburne University of Technology, 2010 31 OF 19 AGE P , occur when the Sun, Earth and the Moon are neap tides Not to scale! HET602-M04A02: Watching the Moon: Time and Tide Thus spring tides occur duringmoon. full and new moons, while neap tides occur at first and third quarter The smallest ocean tides onat Earth, right called angles: c Swinburne University of Technology, 2010 31 OF 20 AGE P HET602-M04A02: Watching the Moon: Time and Tide It’s common for people to referthe to Moon. “the dark side of the Moon”.In But in the fact previous there Activity is we nosame one saw face dark that turned side towards as of us. the (Thethe Moon face month, rotates of as around the Moon we the that shall Earth,the we see Moon it see is in is keeps always not a essentially in actually moment.) the darkness. constant throughout ThusBut people this incorrectly is assume not that thethe case. the month. “other” The side part of of the Moon which is in shadow isn’t constant - it changes during Synchronous rotation of the Moon c Swinburne University of Technology, 2010 31 OF 21 AGE P of the Moon: phases HET602-M04A02: Watching the Moon: Time and Tide The ”dark side” of the Moon isBut that since which we faces see away the from same theside. face Sun. of the Moon throughout the month, there can’t be one constant dark Let’s have another look at the c Swinburne University of Technology, 2010 31 OF 22 AGE P , which means that the rate at which the Moon of the Moon). sidereal period synchronous rotation HET602-M04A02: Watching the Moon: Time and Tide orbits around the Earth israte equal is to 27.3 the days rate long at (which which is it the How rotates does on this its come axis. about?towards For us. the It Moon could this hardly rotation be a coincidence that the Moon keeps the same face This is because the Moon is in c Swinburne University of Technology, 2010 31 OF 23 AGE P System orbit in Solar . : Dr. R. Albrecht, ESA/ESO Space Telescope European Pluto and Charon Credit Coordinating Facility; NASA tidal bulges in synchronous rotation – each with one face eternally turned towards the other. HET602-M04A02: Watching the Moon: Time and Tide both To understand the synchronous rotation oflook the at Moon the (and effect other of satellites), we need to have a closer This is nosynchronous coincidence rotation. – many ofAs the an extreme natural example, and satellites as weare (“moons”) will see in in a the later Activity, far-distant Pluto and its satellite Charon c Swinburne University of Technology, 2010 31 OF 24 AGE P alignment. tidally locked in the form of heat in the crust, taking energy from the Moon’s rotation and friction : Original Moon by: NASA/NSSDC Credit HET602-M04A02: Watching the Moon: Time and Tide therefore slowing its rate of rotation down until it reached its present So as the MoonThis generated rotated, the bulge continually moved through its crust, raising and lowering it. Theories of the formation of the MoonHowever, suggest that the it Moon’s rotated much tidal fasterconnecting bulge in the the Moon due past. and to Earth: the Earth should remain directed (roughly) along a line Tidal locking c Swinburne University of Technology, 2010 31 OF 25 AGE P libration of the same face of the Moon at all times. Because the exactly . HET602-M04A02: Watching the Moon: Time and Tide This allows us to see about1/2 5/8th factor of you the would Moon’s otherwise surface during expect. the This course of slight a “turning” month, of instead the of the Moon is termed the the Moon Moon’s orbit around the Earththe isn’t Moon perfectly during circular, its we orbit. get slightly different views of the surface of As previously mentioned, we don’t see c Swinburne University of Technology, 2010 31 OF 26 AGE P HET602-M04A02: Watching the Moon: Time and Tide We have seen that both Pluto and itsAs satellite our Charon Moon are is tidally locked lockedtidally into into locked synchronous - synchronous rotation. that rotation, is, ittakes why is to the orbit reasonable Earth the to does Earth? ask not why take the the same Earth amount is of not time also to rotate as the Moon The length of the day and the month c Swinburne University of Technology, 2010 31 OF 27 AGE P slowly lengthening due to the effect of the tidal is HET602-M04A02: Watching the Moon: Time and Tide The answer is that thebulge length moving through of the a Earth’s day crust,day on but is the Earth very Earth slow. is so massive thatThe length this of change a in day length on ofEarth Earth the first is formed, increasing the by about day 0.001 may seconds have been perNote, century only however, at 5 present. to that 6 When you the hoursmultiplying long! cannot 0.001 seconds simply by calculate thethe the age Earth-Moon of change distance the changes, in Earth-Moon so too the system does (in length the centuries). rate of at This the which is the Earth’s because Earth’s as day spin slows. by c Swinburne University of Technology, 2010 31 OF 28 AGE P (or twisting force) on the Earth in the opposite sense to its rotation torque HET602-M04A02: Watching the Moon: Time and Tide Since bulge A is closerwhich to results the in Moon a than net bulge B, it feels a great gravitational pull from the Moon, direction, thus slowing it down. At some time in the& distant Charon. future, the Earth-Moon system will also become tidally locked like Pluto Because of the Earth’s relatively rapid rotation, the tidal bulges are dragged off the Earth-Moon line: c Swinburne University of Technology, 2010 31 OF 29 AGE P tidal HET602-M04A02: Watching the Moon: Time and Tide Since bulge A is closerwhich to has the the Moon net than effect bulge over B, time it ofThus pulling exerts the the a Moon greater Moon ahead. gravitational pull slowly on spirals the Moon, outwards away from the Earth. This phenomena is called recession. Let’s have a closer look atshowed the us effect of that the to Earth’s every tidal action bulges there on is the an Moon. equal Remember and that opposite Newton reaction. c Swinburne University of Technology, 2010 31 OF 30 . AGE P year . click here HET602-M04A02: Watching the Moon: Time and Tide The marine record contains evidence ofbeen shorter made that months 2.8 in billions thediscussion years past ago for the the length Earth. of Suggestions the have monthOriginally was the just Moon-Earth 17 days. separation may Fortides have a on been paper Earth as on would little this then as have beenThe one-tenth roughly weathering its one caused present thousand by value. times such greater Ocean huge thanEarth’s tides their surface. present-day would size. have had a profound effect on the evolution of the Currently the Moon is very slowly spiraling outward, increasing its orbital radius at a rate of 4 cm/ c Swinburne University of Technology, 2010 31 OF 31 AGE P acting . tidal recession tidal bulges differential gravitational force . This means that the Moon’s rotation rate is and their effect on both the Earth and the Moon. Tidal of the Moon. libration tidal forces synchronous rotation and hence it always keeps the same face towards the Earth. Because of orbit of the Moon around the Earth, however, we go get to see about 5/8th of period eccentric HET602-M04A02: Watching the Moon: Time and Tide the slightly along the line of centres.side, which The results near in side a of stretching of a that body body will that feel form a (almost) stronger symmetric gravitational pull than the far the lunar surface. This is called the Tidal friction also means that the Moon– is which slowly spiralling increases away the from the length Earth ofEarth’s – a rotational called (lunar) period month. and Tidal hence friction slowing also increasing effects theIn the length the Earth, of next slowly Module, an down we Earth the will day. investigate current theories on the formation of the Solar System. forces act between bodies with a fixed size and result from the In this Activity, we have discussed Summary The tidal forces acting on thethe Moon tidal forces due acting to on the the Earth Earth produceOver due tidal time, to bulges the the in friction Moon the due are to Moon’s theuntil tidal crust, cause forces while it have of taken has the energy become ocean from tides. locked the Moon’s in rotation, slowing it down equal to its orbital